Fuel Tank Pressure (1 Viewer)

[benjamintrd]

I live here in Albuquerque also depending on where you live we go from about 5300-6500 in elevation with summer temps around 90-100. I am new to the 80 but had the same problem with my 100 during the summer I try not to fill the tank all the way up and seems to reduce the boiling venting from the tank. When Maverick gas stations here carried straight unleaded I ran that with no issues so must be something to do with the ethanol heat and altitude. My mechanic is a 80 guy with several of them and told me never to top off the vehicle. 2 summers later and I don't get the venting or boiling in my 80 but I never fill it all the way up just around 3/4 of a tank.

 

[WCBlueSky]

Yeah interesting, I also have new cat. Works great and doesn't get nearly as hot as the OEM.

Nice. I'm planning to measure the exhaust back-pressure to see if mine are clogged. If the engine or exhaust is hotter than normal I could see that heat transfering into the gas tank and adding heat shield on the exhaust may help. One time when the gas tank was boiling I used an infrared thermometer to measure the outside temperature of the gas tank, it was 130 deg F as was everything else (or higher) in the heat-soaked under carriage. I wonder if the gasoline inside the tank was the same temperature or even hotter. If the gas in the return line is super hot and heating up the tank then maybe putting a cooler or insulation on the fuel line would help.

 

[doc_random]

Here is a link for boiling points.

Fuels - Boiling Points

Fuels and their boiling points.

www.engineeringtoolbox.com

Ethanol boils at 172'F, Octane boils around 250'F. The partial pressure of ethanol will make the tank boil much sooner with an ethanol blend.
The gas cap keeps some pressure in the tank which will increase the boiling point (delay boiling). When we vent a hot tank it lowers the boiling point which is why there seems to be a ridiculous about of venting because its boiling. Probably better not to vent (assuming your canister is working properly).
Driving downhill or at speed should cool it off.
Higher altitudes lowers the boiling point as well. When I go from Lake Pleasant (1700' elevation) to Crown King (5770' elevation), uphill, in the summer, the gas gets pretty hot. Pretty much what the OP wrote.

Maybe the 3/4 full trick makes you fill up more often, adding relatively cool gasoline to the hot tank?
With heat soak I don't know if there is any point to insulating the gas lines in the engine bay. Has anyone tired it?

 

[LandLocked93]

I run eth-free fuel exclusively. Still alot of venting thru the cap in conducive situations.

 

[enox]

Update - So I think the Fuel contents along with high heat/ elevation/ full tank/ low speed all contributed to the fuel boiling. The new VC120 has the same resistance as the one I have had for 2 or so years.

Also there was another 80 on the same trip, who experienced the same venting/ boiling. There was 1 jeep in our group too, who did not have a venting issue or boiling gas.

 

[LandLocked93]

There was 1 jeep in our group too, who did not have a venting issue or boiling gas.

...aren't most Jeeps' muffler and gas tank right next to each other?
If not, even by square area aren't they closer to each other than they are on an 80?
Just trying to determine the source of the heat to the fuel.
Kinda feel that heat is not part of the issue imho.
Only way to find out I guess is to put a temp probe into a boiling tank.

 

[BILT4ME]

...aren't most Jeeps' muffler and gas tank right next to each other?
If not, even by square area aren't they closer to each other than they are on an 80?
Just trying to determine the source of the heat to the fuel.
Kinda feel that heat is not part of the issue imho.
Only way to find out I guess is to put a temp probe into a boiling tank.

Realize that the LC's have a Fuel Return Tube that circulates unused fuel back to the tank during operation. So, for low speed hill climbing, the engine heat increases, increasing the temperature of the fuel as it is recirculated through the system.

Couple that with heat of compression of the pump compressing the fuel for pressure at the injectors, you get a lot of heat buildup in the fuel tank.

Some of us old folks know about Vapor Lock and the old cars that would get the fuel lines hot and the car would stall because the lines got hot enough to vaporize the fuel in the lines and the carburetor no longer "worked".

Same principle here, and, because I own an OLD carbureted car, I also know that ethanol fuel is much less tolerant of heat than non-ethanol fuel. My Studebaker will vapor lock when the engine gets hot and I'm not circulating enough air. It is WAAAAAAY worse with ethanol fuel and happens at much lower temperatures.

Couple that with a drastic elevation change (vapor pressure is lower at altitude, so things boil at lower temperatures)

So, It's possible that the jeeps don't have that issue because their entire fuel system is DESIGNED to be under a higher pressure or vacuum. Just like a radiator, you increase the pressure, you increase the boiling point. Works the same for the fuel.

The LC is kind of a crossover point between old and new (it was really new at the time!) and design parameters have changed since then, as fuels have changed and conditions have influenced design.

Many of the new cars REQUIRE a sealed fuel system in order to keep it under pressure during operation. The car may not even run if the gas cap is off.

 

[LandLocked93]

Realize that the LC's have a Fuel Return Tube that circulates unused fuel back to the tank during operation. So, for low speed hill climbing, the engine heat increases, increasing the temperature of the fuel as it is recirculated through the system.

Couple that with heat of compression of the pump compressing the fuel for pressure at the injectors, you get a lot of heat buildup in the fuel tank.

Some of us old folks know about Vapor Lock and the old cars that would get the fuel lines hot and the car would stall because the lines got hot enough to vaporize the fuel in the lines and the carburetor no longer "worked".

Same principle here, and, because I own an OLD carbureted car, I also know that ethanol fuel is much less tolerant of heat than non-ethanol fuel. My Studebaker will vapor lock when the engine gets hot and I'm not circulating enough air. It is WAAAAAAY worse with ethanol fuel and happens at much lower temperatures.

Couple that with a drastic elevation change (vapor pressure is lower at altitude, so things boil at lower temperatures)

So, It's possible that the jeeps don't have that issue because their entire fuel system is DESIGNED to be under a higher pressure or vacuum. Just like a radiator, you increase the pressure, you increase the boiling point. Works the same for the fuel.

The LC is kind of a crossover point between old and new (it was really new at the time!) and design parameters have changed since then, as fuels have changed and conditions have influenced design.

Many of the new cars REQUIRE a sealed fuel system in order to keep it under pressure during operation. The car may not even run if the gas cap is off.

Affirmative on the return line but (and just trying to get a grip on the concept) by volume wouldn't it have more surface cooling area compared to the fuel rail on the way back to the tank? It is also half or less the diameter (and 10x longer) than the rail, so how much fuel (and how warm) by volume is actually returning?
In any scenario? In stop and go traffic on north-loop 285W in ATL in August @4pm on a Friday, there's not a trail run you could think of that could be hotter. lol At most this developed the 'whoosh', but no boiling ime. This seems to suggest elevation plays more a part than heat imo.

Fascinating the new stuff out. Hard to keep up. I recall reading about 20kpsi fuel systems on newer vehicles which sounds insane. And the diff between today's and yesterday's fuels is evidently far greater than we consumers understand by and large.
But didn't connect that newer vehicles were developed to use the newer fuels differently.
Makes perfect sense.

 

[BILT4ME]

Affirmative on the return line but (and just trying to get a grip on the concept) by volume wouldn't it have more surface cooling area compared to the fuel rail on the way back to the tank? It is also half or less the diameter (and 10x longer) than the rail, so how much fuel (and how warm) by volume is actually returning?
In any scenario? In stop and go traffic on north-loop 285W in ATL in August @4pm on a Friday, there's not a trail run you could think of that could be hotter. lol At most this developed the 'whoosh', but no boiling ime. This seems to suggest elevation plays more a part than heat imo.

Fascinating the new stuff out. Hard to keep up. I recall reading about 20kpsi fuel systems on newer vehicles which sounds insane. And the diff between today's and yesterday's fuels is evidently far greater than we consumers understand by and large.
But didn't connect that newer vehicles were developed to use the newer fuels differently.
Makes perfect sense.

When you're climbing a mountain, you are at a higher throttle position that idling at a standstill.
Climbing the mountain, your air is getting thinner so your "cooling" will be decreased.
At idle, you're returning more fuel to the tank, so it has less retention time in the lines to GAIN heat. The fuel pump is also running slower (two-stage pumps)
At higher throttle position, you're using more fuel, generating more heat, less fuel is returning, so there is more line retention time to absorb heat. There is continuously less fuel to absorb the heat brought back so it increases the overall temp faster.

I agree, heat in ATL rush hour is brutal. Taking it to the mountain with lower vapor pressure and working it harder does not average out the enthalpy of the two scenarios.

 

[enox]

Went out yesterday doing some crawling in the heat, again I had like 3/4 tank of fuel and did have a lot of pressure build up every time I stopped and released the cap.

I can also hear that the canister is working as it's venting from there too.

I guess, as long as the canister isn't clogged and it's venting at some capacity then the tank isn't at risk of popping a seam?

So even though pressure builds up, it is venting, just not enough to keep it from ever having a pressure issue. Then again I assume it's just the heat causing it to happen..So maybe it's not an issue even during these summer months?

 

[Road Apple]

Replaced my canister with a VC120 last month. Before replacement I had a lot of pressure release at sea level driving.

Left Sacramento yesterday, 315’ elevation, high 90 degree temps, with a full tank and stopped in Arnold, CA, 4000’ elevation, to top up before hitting the trail. The fuel had ethanol. The tank took 10 gallons. No pressure release when I took off the cap.

Crawled Slick Rock, elevation 7000+, for 4 hours with 85 degree temps, and then back to Sacramento. Opened the tank with no pressure release. I didn’t hear fuel boiling on the trail. The vapor recovery system and the VC120 are working so far. Will keep an eye on it though.

Thanks for posting @enox. Hope your troubles go away with cooler temps and you can find the issue soon.

 

[enox]

Replaced my canister with a VC120 last month. Before replacement I had a lot of pressure release at sea level driving.

Left Sacramento yesterday, 315’ elevation, high 90 degree temps, with a full tank and stopped in Arnold, CA, 4000’ elevation, to top up before hitting the trail. The fuel had ethanol. The tank took 10 gallons. No pressure release when I took off the cap.

Crawled Slick Rock, elevation 7000+, for 4 hours with 85 degree temps, and then back to Sacramento. Opened the tank with no pressure release. I didn’t hear fuel boiling on the trail. The vapor recovery system and the VC120 is working so far. Will keep an eye on it though.

Thanks for posting @enox. Hope your troubles go away with cooler temps and you can find the issue soon.

Thanks for the data point!

Yeah interesting, I don't get any pressure home at sea level just up in the mtns with hot temps.

Maybe I will swap a new vc120 just for fun and see if it doesn't happen next time I'm up on the rocks...

 

[Broski]

What shape is your gas cap in?
I had that very same experience 3 years ago. Stopped for a break and heard cracks and pops coming from the fuel tank area under the truck. Cracked the fuel cap and fuel vapors and liquid came flying out.
Immediately went thru the canister cleaning the ports and check valves. Never had another issue. Then I replaced the fuel filler cap early this spring as PM.

And THEN...one day last month I was standing near the drivers-side rear taking some pics and hear it...a slow, lengthy, barely audibly exhale. Like the kind one does before 'squeezing a trigger'. It stopped for 10 seconds, then did it again. Then stopped, then again. I'm usually solo on my rides, so the sound was unnerving but decided it was my imagination that 'Squatch' was just behind a nearby tree.

Finally, on the way back down from a trip to 12kft, I stopped for lunch and hear it again. This time zeroed in on the fuel filler and popped the door open to check. Sure enough, it was venting from the tank thru the cap...as it should do presumably...in 10 or so second intervals. This went on for 20 minutes or so before it eventually quit.

Guessing a properly operating system includes a good cap and canister resulting in venting both back thru the can and thru the cap

I've had a VC120 canister in my 80 for at least 5 years. I was noticing the ethanol smell that I used to get from the original OEM canister. I swapped to a brand new VC120 and still have the smell. I attribute the smell and the increased pressure to the increase in ethanol in the gas. In 2019, the EPA allowed the summer blend to go from %10 to %15 ethanol. You might try a tankful of non-ethanol gas to see if the pressure drops noticeably.

This if possible don't run ethanol

 

[White Stripe]

I haven't ran a stock evap system for a while now and I don't remember what all it consists of. With my v8 I installed a gm evap system. It has a vent and purge solenoid. If those aren't working the fuel system won't get rid of excess vapors. Does the stock Toyota evap system have solenoids on it? If so how old are they? I've noticed the new 5TH gen 4runners have very similar undercarriage designs as a 80 with exhaust on passenger side of driveshaft and fuel tank on driver side of driveshaft. However on the 5TH gen Toyota has a fair bit of heat shielding around the fuel tank. I'm going to be installing similar heat shielding around my fuel tank. Going up Hills makes the motor work harder, thus the exhaust gets hotter, and so the radiant heat from the exhaust heats up the fuel tank more. This is the type of stuff I'm thinking of using...

Embossed Aluminum Heat Shield, 20" x 28"

Keep engine bay heat from entering the cabin of your vehicle with the Mishimoto Embossed Aluminum Heat Shield. With the use of heavy-duty aluminum, this heat shielding is ready to defend against continuous temperatures up to 1100°F. The 20”x28” sheet of embossed aluminum can be easily...

www.mishimoto.com

 

[enox]

Yeah I wonder if it will be an issue if I switched to a long range fuel tank.

 

[b16]

Great timing on this thread. Had the same issue coming from CA to Moab UT. Was 106 in Moab and had the same thing. Vc120 is only a couple years old and had severe pressure when I went to fill up. Engine was working hard at altitude, heat and AC on. My cooling system should be in good shape. Modded blue hub clutch fan, recent flush and new t-stat. My A/C would cut off on some of the harder pulls on the freeway. The 80 struggles to stay near 80mph on i70 with 35’s and stock gears. Lol.

went into rabbit holes on the issue regarding boiling. Seems my vc120 is operating as expected. Curious to see if your new one helps.

I replaced my vacuum lines to and from the canister while up here in CO as they were super hard and some cracked. Hope it helps. Will know more next weekend when I drive back to CA. Might hit Imogene pass on Sunday.

 

[enox]

Doesn't the "purge" side of the vapor canister, hook up to that VSV that's underneath the plenum?

I wonder if that is the issue..

 

[Shoredreamer]

I think another factor that plays into the tank pressure/boiling is your engine RPMs. If I remember, the VCV switches at 3000rpms to open the Evap to intake vs being absorbed into the CC.

This makes sense in that the most common time I get tank pressure is when I am doing a climb on a trail 2000’+ where rpms stay low. A hot day doesn’t help the situation also.

Also curious as to what % of ethanol is in the gas and it that varies by brand or state?

 

[BILT4ME]

Has anyone tried this with leaving the gas cap loose?

 

[BILT4ME]

I guess, as long as the canister isn't clogged and it's venting at some capacity then the tank isn't at risk of popping a seam?

Pressure / vacuum cycling the fuel tanks is what causes them to fail on the top of the tank at the top of the tank, near the front, on the bend where the tank was pressed into shape.

If the tank creates too much vacuum then it sucks in, then as you run it, it creates pressure and the tank expands, so the formed area fatigues over time, causing a failure.

This can definitely be a problem. If you have not read up on it, find the threads on here about cracked fuel tanks.